White spot disease update

The arrival in Australia of a highly contagious crustacean disease has prompted an emergency response to protect the prawn industry

Until late 2016, Australia was one of two remaining prawn farming countries in the world that were free of white spot disease.

However, on 22 November, 2016, potential signs of white spot disease were detected on the first farm in Australia and, subsequently, on 30 November, Queensland’s Biosecurity Sciences Laboratory identified the white spot syndrome virus. Samples were sent to the Australian Animal Health Laboratory in Geelong and it confirmed these findings on 1 December.

The prawns affected were farmed Black Tiger Prawns (Penaeus monodon).

White spot disease is a highly contagious viral disease that affects prawns. It is caused by a virus known as white spot syndrome virus. The virus that causes it is exotic to Australia and poses a significant threat to Australia’s farmed crustacean industries.

Since the initial infection, the disease has spread in South East Queensland. Additionally, about 100 infected Black Tiger Prawns were found in the wild, outside the affected farms, in February.

Treatment work is ongoing across all infected properties to ensure the virus is destroyed as quickly as possible. This includes the gradual and methodical draining, drying and clearing out of all sediment from farms that have already been decontaminated. This work is expected to take a number of months to complete.

To date, about 3.8 million litres of chlorine have been used to treat ponds, water channels and settlement ponds. This is the largest emergency aquatic animal disease response ever in Queensland. The Queensland Government has spent $4.4 million on disease control activities, including more than 100 staff members and more than 50,000 laboratory tests to detect the spread of the disease.

Research needs

Since the outbreak was detected, both government and industry have initiated research programs to better understand it. The primary focus of research is testing and analysing infected prawns and pathways for transmission, with a view to stopping further transmission and, ultimately, eradication.

However, should the white spot virus be found to persist in Australian waterways, research will focus on addressing how to minimise the impact on the fishing and aquaculture sectors. Should the disease become entrenched, the research strategy will be to focus on selective breeding, diagnostics, biosecurity and treatment.

To date, scientific evidence shows that white spot disease has not established in the wild, demonstrating that current biosecurity and eradication measures have been effective in keeping the virus contained. Should future testing fail to detect the virus in the wild, research and monitoring will need to be undertaken to document and justify the reinstatement of Australia’s white spot disease-free status. A Senate inquiry has been announced to investigate how white spot disease entered and infected Australian prawn farms.

About the disease

It is generally found in semi-tropical and tropical zones; there is little evidence of infection in temperate waters.

In farms it is known to have a mortality rate of more than 90%.

White spot disease is widespread throughout prawn farming regions in Asia and the Americas, where it has caused severe losses.

Other crustacean species can be infected and also act as carriers.

Based on experience in other countries, the impact on wild stocks is markedly different to that of farmed prawns. While present in
the wild overseas, the virus has not had an impact on mortality or productivity.

Freezing does not kill the virus, but cooking does.

Transmission of the virus can occur in a number of ways – from crustacean to crustacean (when one prawn eats another), through contaminated raw prawn products, from contaminated feed including probiotics, from brood stock which contains the virus, or by human transmission via gear such as aerators or shoes.

Terrestrially the virus can be spread by birds, crabs, water rats and some land-based animals.

Human impacts

The disease does not pose a threat to human health or food safety. It is important to note that affected prawns will not enter any domestic or international markets or be processed for human consumption. It is crucial that people fishing or crabbing in any of Australia’s waterways do not to use prawns meant for human consumption as bait. This has the potential to spread the virus.